Serotonin (5HT) is hypothesized to be a contributor to developmental programming that results in increased risk for childhood and adult-onset mental illnesses. This may occur through the capacity of 5HT to serve as a modulator of processes that include cell proliferation, migration and circuit wiring. Until recently, it was thought that 5HT sources impacting brain development arose from maternal transport of 5HT to the fetus, or from raphe neurons in the brainstem of the fetus. We recently reported that 5HT during pregnancy does not arise from the mother, but rather is synthesized in the human and mouse placenta from maternally-derived tryptophan. Moreover, 5HT accumulates selectively in the fetal forebrain from e10.5-e15.5, but not the hindbrain. These data collectively suggest that developing circuits involved in mediating mood and emotion are sensitive to extraembryonic 5HT during initial ontogeny, which could be impacted by both genetic and environmental disturbances that are known to increase risk for mental illnesses. Specifically, because the developing fetus and placenta are genetically identical, the polymorphisms impacting the 5HT system that are associated with increased risk for mental health-related psychiatric disorders, as well as adverse environmental stressors, could be impacting both organs. The relative contributions of placental and fetal 5HT sources to long-term outcome of offspring are unknown, and will be explored in Project 2, Influence of Placental Serotonin on Neural Development and Behavior. Here, a strategy of selective genetic manipulation of 5HT in the placenta is advanced. The proposed studies take advantage of a placenta-specific promoter of the CYP19 gene, driving Cre expression, only in the placenta, for deleting genes that are engineered to have LoxP sites. In Aims 1 and 2, the rate-limiting enzyme for 5HT synthesis in the placenta, tryptophan hydroxylase 1, or the principle metabolic enzyme, monoamine oxidase A, will be deleted. The impact of either eliminating or greatly increasing placental 5HT on forebrain and hindbrain neurochemistry and receptor development and basic forebrain patterning will be examined using neurochemical, bioanalytical and molecular neuroanatomical approaches overseen by the Conte cores. As an additional approach, the disruption of the 5HT transport via deletion of placental serotonin transporter (SERT) will be achieved with development of a new floxed SERT animal model that will be generated in the context of Project 3. The enduring influence of genetically altered placental 5HT on emotional regulation will be examined in Aim 3. The studies will, for the first time, define the placental extraembryonic contributions of 5HT to the assembly and tuning of central neurochemical systems and impact on basic developmental processing that are sensitive to 5HT signaling prenatally.